The Prime Minister has also failed to commit to the UK remaining a ‘tier-one’ military power after leaving the EU next March.

And Dr Tara McCormack, a politics lecturer at the University of Leicester, backed Mrs May’s pragmatic stance on defence.

She said: “Theresa May is entirely right to say that defence policy needs justification.

“Brexit can and should be an opportunity for Britain to have a serious debate about our foreign and security policies and our underlying national interests.

“Unfortunately, at the moment, British politicians and the military establishment are like cyclists who daren’t stop pedalling to actually think about their actions – beyond a fixation with maintaining Britain’s status as a big military power.

“Aside from doubling down on nostalgia for when Britain ruled the waves, we need to step back and think much more seriously about what we want to do and be in the world.

“The debate about British foreign and security policy, and the underlying question about our national interests, has been preserved in aspic since the end of the Second World War.

“We need an honest discussion and also an honest evaluation of post-Cold War British foreign policy.”

Dr McCormack said Britain’s foreign policy since the 1980s had been “extremely destabilising”, especially in Iraq, Libya and Syria.

Writing on the London School of Economics’ Brexit blog, the academic added: “Britain needs to have a serious democratic debate about foreign policy and the role that we want our country to play in the world.